I'm not sure what your first code block has to do with add a DOCPROPERTY field to a document. The correct syntax to add a DOCPROPERTY field is:
Code:
ActiveDocument.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
Text:="DOCPROPERTY FirstTest1Custom", PreserveFormatting:=True
Note that you'll get an error output if the FirstTest1Custom Custom Document Property doesn't exist.